In-depth Understanding of Polarity for MTP System

To meet increasing demands for high-density cabling and wider bandwidth of network applications, many data centers are migrating to the 40G and 100G Ethernet. To prepare for this change, MTP technology is applied to provide an easy migration path. Typically, a fiber optic link needs two fibers for full duplex communications. Thus the equipment on the link should be connected properly at each end. However, high-density connectivity usually requires more than two fibers in a link, which makes it more complex to maintain the correct polarity across a fiber network, especially when using multi-fiber MTP components for high data rate transmission. This article will specifically guide you to understand the polarity for MTP system and three MTP polarity methods.

What Is Polarity?

To form a fiber optic link, the optical transmitter at one end is connected to the optical receiver at the other end. This matching of the transmit signal (Tx) to the receive equipment (Rx) at both ends of the fiber optic link is referred to as polarity. In other words, polarity is the term used in the TIA-568 standard to explain how to make sure that proper connection is made between the transmitter at one end and the receiver at the other end. Once the component is connected to the wrong polarity, the transmission process will be unable to go on.

Structure of MTP Connector

As shown in the following picture, MTP connector is pin and socket connector, which requires a male side and a female side. And each MTP connector has a key on one side of the connector body. When the key sits on top, this is referred to as the key up position, and when the key sits on bottom, we call it key down position. Moreover, each of fiber holes in the connector is numbered in sequence from left to right. We will refer to these connector holes as positions, or P1, P2, etc. Besides, each connector is additionally marked with a white dot on the connector body to designate the position 1 side of the connector when it is plugged in.

Structure of MTP Connector

MTP Adapter Keying Options

MTP adapter contains an asymmetrical housing including an inverted key to achieve the appropriate fiber polarity. On type A adapters, the keys are inverted to ensure that the fiber at position 1 is connected to position 1 in the MTP fiber cable connector at the opposing end.

MTP key up to key down adapter

On type B adapters, both keys are oriented facing up in order that both MTP fiber cable connectors are mated “key up”. The fiber at position 1 is connected to position 12 in the MTP connector at the opposing end.

MTP key up to key up adapter

Three Polarity Methods for MTP System

The TIA standard defines two types of duplex fiber patch cables terminated with LC or SC connectors to complete an end-to-end fiber duplex connection: A-to-A type patch cable is a cross version and A-to-B type patch cable is a straight-through version. Based on this, there are three polarity connecting methods for MTP system. The following part will introduce them in details.

A-to-A type patch cable and A-to-B type patch cable

Method A

Method A utilizes “key up to key down” adapters to connect the MTP connectors. As the following figure shows, this method maintains registration of Fiber 1 throughout the optical circuit. Fiber 1 in the near end cassette mates to Fiber 1 in the trunk cable assembly, which mates to Fiber 1 in the remote cassette. The fiber circuit is completed by utilizing one flipped patch cord, either at the beginning or end of the permanent link, to insure proper transceiver orientation. Method A provides the simplest deployment, and works for single-mode and multimode channels, as well as can easily support network extensions.

Method A

Method B

Different from method A, method B uses “key up to key up” adapters. The fiber circuit is completed by utilizing straight patch cords at the beginning and end of the link, and all of the array connectors are mated key up to key up. This type of array mating results in an inversion, meaning that Fiber 1 is mated with Fiber 12, while Fiber 2 is mated with Fiber 11, etc. To ensure proper transceiver operation with this configuration, one of the cassettes needs to be physically inverted internally so Fiber 12 is mated with Fiber 1 at the end of the link. This method requires a more in-depth planning stage in order to properly manage the polarity of the links, and to identify where the actual inversions need to occur. Moreover, it only supports multimode fiber.

Method B

Method C

With the use of “key up to key down” adapters, method C looks like method A. However, the difference between method C and method A is that the flip does not happen in the end patch cords, but in the array cable itself. This method requires a more in-depth planning stage in order to properly manage the polarity of the links, and to identify where the actual flipped array cord is placed in the link. An additional drawback to this method is that if this link was to be extended, a straight array cord as used in Method A would need to be used to revert the polarity back to straight array polarity condition. In other words, unflip the array cable.

Method C


Knowing the polarity of MTP system helps you better upgrade the 40G and 100G networks. According to different polarity methods, choosing the right MTP patch cables, MTP connectors and MTP cassettes will provide greater flexibility and reliability for your high-density network.

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